sign in sign up
<<
Home , Phototropism, Polar auxin transport

7

Auxin transport Ð shaping the JirÏõÂ Friml

Plant growth is marked by its adaptability to continuous changes answer to their sessile fate and which became their major in environment. A regulated, differential distribution of adaptation strategy. Special tissues have underlies many adaptation processes including organogenesis, evolved, which maintain the ability of plant cells to divide meristem patterning and . In executing its multiple roles, and differentiate throughout the of the plant, and a auxin displays some characteristics of both a hormone and a number of differentiated cells keep their potential to morphogen. Studies on auxin transport, as well as tracing the elongate, forming the basis of plant tropisms. Thus, intracellular movement of its molecular components, have can ¯exibly change their shape and size to optimally suggested a possible scenario to explain how growth plasticity is adjust themselves to a changing environment. During conferred at the cellular and molecular level. The plant perceives the past century, an endogenous plant signal, auxin, and stimuli and changes the subcellular position of auxin-transport its distribution in the plant have been increasingly estab- components accordingly. These changes modulate auxin ¯uxes, lished as playing a central role in these complex adapta- and the newly established auxin distribution triggers the tion responses. Recently emerging molecular data have corresponding developmental response. shed light on the mode of auxin action and its regulated transport, and have begun to connect the plasticity of Addresses whole- with processes at the cellular Zentrum fuÈ r Molekularbiologie der P¯anzen, UniversitaÈ tTuÈ bingen, Auf level. der Morgenstelle 3, 72076 TuÈ bingen, Germany e-mail: [email protected] A century towards molecular players Our linden tree had to wait more than 300 years before our Current Opinion in Plant 2003, 6:7±12 curiosity was turned towards the mysterious mechanisms that so strangely affected its fate. At that time, a German 1369-5266/03/$ ± see front matter botanist, Julius Sachs, proposed the existence of signaling ß 2003 Elsevier Science Ltd. All rights reserved. substances that regulate coordinated plant growth, and DOI 10.1016/S1369-5266(02)00003-1 Charles Darwin together with his brother Francis grew grass in unilateral light to demonstrate the Abbreviations existence of a transported signal that mediates plant AUX1 AUXIN1 [1,2]. Thus, the history of auxin began. IAA indole-3-acetic acid During the century that followed, the chemical nature MDR multidrug resistance of auxin was uncovered, but we remained confused by the PIN PIN-FORMED SGR2 SHOOT GRAVITROPIC2 variety of apparently unrelated developmental processes ZIG ZIGZAG that are regulated by such a simple molecule as indole-3- acetic acid (IAA).

Prelude Ð The linden tree of innocence Our attention was drawn to the transport of auxin, as its On the margin of the ChirÏiby hills, an old mediaeval castle disruption interferes with almost all auxin-related devel- `Buchlov' guards the wide valley of the South Moravian river opmental processes. We learned that auxin can be dis- `Morava'. There, on the terrace where the tribunal of the hunter's tributed via the phloem or by a directional, so-called court used to sit, and where the last farewells with the convicts `polar', transport system (see Figure 1; [3]). The large were held, a famous linden tree of innocence stands as a witness amount of physiological and biochemical data on polar of a local legend. It is told that early in the 16th century the lord auxin transport has been integrated into the `chemiosmo- of the castle was deceitfully slain during one of his frequent hunts. tic hypothesis'. This classical model explains the -to- A young servant was accused of this murder and imprisoned. cell movement of auxin by the action of speci®c auxin- After long days of unavailing torture he was condemned to death in¯ux and -ef¯ux carriers. The asymmetric positioning of on the castle terrace. At this, the young man rose and pulled out the latter at a particular side of the cell was proposed to the young linden tree growing nearby. He set it inverted back into determine the direction of auxin ¯ux [4,5]. This model the soil with the words, ``If next year this small tree will grow was reinforced by the identi®cation and characterization green, it will be a sign of God that you killed an innocent''. And of candidate proteins for auxin in¯ux (AUXIN1 [AUX1]/ indeed, in the spring, small green ¯ourished from the LIKE-AUX1 [LAX] family) and ef¯ux (PIN-FORMED previous and the young man was set free. [PIN] family) carriers [6±8]. Numerous pieces of circum- stantial evidence demonstrate the role of these proteins in This rather romantic story demonstrates the fascinating auxin transport despite the lack of rigorous proof for their plasticity of plant growth, which plants developed as an function as carriers [3]. PIN proteins are asymmetrically www.current-opinion.com Current Opinion in Plant Biology 2003, 6:7±12 8 Growth and development

Figure 1 lamic acid (NPA), an inhibitor of auxin transport, thus providing an additional connection between MDRs and auxin delivery [14]. Although detailed information is still lacking, a century of studies on auxin transport have brought us the identity of a couple of players that are involved in auxin responses, and an image of a complex network of several transport systems that are involved in distributing auxin throughout the whole plant.

Hormone or morphogen? Two theoretical concepts have greatly in¯uenced the way that we think about auxin and its action: the concepts of a hormones and a morphogen. The mammalian hormone concept de®nes hormones as extracellular signaling mole- cules, which act on target cells distant from their localized site of synthesis [15]. Although recent studies have demonstrated the potential for IAA synthesis in a variety of Arabidopsis organs, its movement from its main source in young apical tissues throughout the whole plant has been proven many times [3,16]. A known role for auxin in coordinating the development of organs, for example lateral roots, with the developmental stage of the shoot provides a functional meaning for this long-distance sig- naling [17,18]. Thus, auxin formally adheres to the clas- Gravity sical de®nition of a hormone. However, the most-studied form of auxin transport, cell-to-cell polar transport, con- trasts with the passive allocation of animal hormones through blood, which is more analogous to non-polar auxin distribution. Several arguments indicate that Current Opinion in Plant Biology non-polar transport in phloem contributes to the move- ment of auxin from its main source in the apical tissues to Auxin response and transport in a gravistimulated Arabidopsis hypocotyl. Auxin transport throughout the plant involves both non-polar the . First, the known velocity of transport in phloem (dashed line) and an active, cell-to-cell polar (about 10 mm per hour) is too slow to execute ef®cient transport (red arrows), which can transport auxin either basipetally signaling, especially in larger plant species. Second, free (from the apexto the base) or laterally. During gravitropic or phototropic auxin has been detected at relatively high concentrations bending, increased auxin response (as indicated by DR5::GUS, displayed as blue staining) corresponding to higher auxin levels is found (of about 1 mM) in phloem exudates [19]. And third, aux1 in the more elongated, outer side of bending hypocotyl. This mutants, which are apparently impaired in loading auxin asymmetric lateral auxin distribution appears to be established by from leaves into the phloem and in unloading auxin from lateral auxin transport and to trigger asymmetric growth. phloem into the root, display defects in their ability to distribute auxin between the shoot apex and the root [12,20]. Thus, the putative auxin permease AUX1 localized in different cells, and this localization impres- seems to act at both ends of the auxin route in phloem, sively coincides with the known directions of auxin ¯ux connecting it at its lower end to the polar transport system [8,9,10,11]. The AUX1 protein also localizes asymme- in the root tip. trically in root protophloem cells at the opposite cell side from PIN1 [12]. These ®ndings suggest that, at least in In the root meristem, auxin is implicated in regulating the some tissues, in¯ux and ef¯ux carriers in concert facilitate pattern of cell division and differentiation (see Figure 2), the vectorial movement of auxin. Recently, members of a short-distance activity that is related to the role of auxin another protein family, namely multidrug resistance as morphogen. The term morphogen was introduced as a (MDR)-type ATP-binding cassette (ABC) proteins, have purely theoretical term in mathematical models of self- been implicated in auxin transport. Two of these proteins, organizing systems, and has evolved into a basic concept AtMDR1 and P-glycoprotein1 of [21]. The least stringent de®ni- (AtPGP1), were originally identi®ed as being functionally tion refers to a morphogen as a substance that forms a related to anion channels; nonetheless, the corresponding concentration gradient and is involved in developmental mutants and double mutants showed auxin-related phe- patterning [21]. More rigorous de®nitions provide three notypes including a reduced rate of auxin transport [13]. critical conditions that bona ®de morphogens must meet: Moreover, these proteins can bind 1-N-naphthylphtha- ®rst, a morphogen forms a stable concentration gradient;

Current Opinion in Plant Biology 2003, 6:7±12 www.current-opinion.com Auxin transport Friml 9

Figure 2 How does the auxin gradient arise? In the root, the auxin ef¯ux regulator PIN4 is asymmetrically distributed towards cells with increased DR5 response, and both Epi pin4 mutations and the chemical inhibition of auxin Cor Ste transport disrupt the distribution of DR5 activity End [11,26]. These ®ndings suggest that PIN4-dependent ef¯ux-driven auxin transport actively maintains the auxin gradient. It is intriguing to speculate that auxin, in turn, in¯uences the position and activity of the auxin ef¯ux carriers; and thus, that the auxin gradient is stabilized by a feedback loop. But can such a gradient instruct pattern- LRC QC LRC ing? The endogenous application of or auxin- Ci transport inhibitors, as well as the use of mutants that are impaired in auxin response or transport, have been C used to establish a link between auxin distribution and patterning [11,26]. Interestingly, changes in cell fate (inferred from the cell-speci®c markers) spatially corre- C late with changes in auxin gradients [11]. Nonetheless, we know too little about auxin or auxin-gradient percep- tion and downstream signaling to be able to pinpoint the direct connection between auxin gradients and instructed C cells. Increasing intracellular auxin content by IAA treat- ment interferes much less with patterning than does the application of non-transportable 2,4-dichlorophenoxyace- tic acid (2,4D). This may even mean that relative differ- ences in auxin content between cells, rather than the absolute amount of intracellular auxin, are instructive. In this scenario, components of auxin transport such as PIN Current Opinion in Plant Biology proteins might serve as auxin ¯ux `counters'. Thus, our knowledge, especially on the interpretation of auxin Pattern of different cell types in a lugol-stained Arabidopsis root gradients, is too scarce to allow us to decide whether meristem. The quiescent center (QC) in the middle is surrounded by auxin is a true morphogen, although it meets several of undifferentiated initial cells, such as columella initials (Ci) that give rise to differentiated lugol-stained layers of columella (C). Differentiated cell the descriptive criteria. Our notion of transport-driven types such as (Epi), cortex(Cor), endodermis (End), stele auxin gradients also contrasts with the classical image of (Ste) and lateral root cap (LRC) are indicated. The regular and invariant morphogens freely diffusing from a source [11].Itis pattern of cell-fates correlates with the auxin gradient, which reaches its therefore questionable how much can we gain from maximum in the columella initials. grafting concepts that are derived from different experi- mental systems onto a plant-speci®c situation. Currently, the morphogen concept is also being revised in the animal ®eld. In planar transcytosis models, morphogens such as second, it instructs responding cells directly (not through decapentaplegic (DPP) or Wingless (WG) move actively another signaling pathway or by signal relay); and third, a through a ®eld of cells and their gradient is maintained cell's response to a morphogen is dependent on morpho- by vesicular traf®cking [23]. It may well be that, as our gen concentration [22,23]. Does auxin ful®ll these experimental knowledge of both auxins and the morpho- criteria? Using new analytical tools, a graded distribution gens increases, the theoretical concepts will converge and of free auxin (i.e. an auxin gradient) has been demon- we will reach a common understanding about pattern strated, for example, in Scots pine or along the Arabidopsis formation in both plants and animals. root [24,25]. Similarly, auxin reporters that are based on auxin-inducible promoters (e.g. DR5) have been used to Tropisms, transport, and traf®c indirectly con®rm the existence of an auxin gradient Another process that in¯uences plant shape is directional within the root meristem, with its maximum in the bending with respect to an exogenous stimulus (mainly to columella initial cells (see Figure 2; [26]). The use of light or gravity), which is called . The role of auxin these reporters may be inadequate, however, especially in tropisms was implied by the Cholodny±Went hypoth- when the throughput of the auxin signaling pathway esis, which suggests that unequal distribution of auxin becomes limiting; nonetheless, in tested situations, their between the opposite sides of a curving organ underlies activity correlates with direct auxin measurements differential growth, resulting in bending [27]. Differential [11,25]. auxin or auxin-response distribution within various www.current-opinion.com Current Opinion in Plant Biology 2003, 6:7±12 10 Growth and development

organs, with higher levels at the lower or less-illuminated Figure 3 side, has been correlated to the their bending in various experiments (see Figure 2; [3,7,10,28,29]). Nonetheless, the question of how this asymmetry is achieved remains unanswered. Went originally proposed that cells change their polarity, which results in the lateral transport of auxin [27]; and experiments in which auxin ef¯ux has been disrupted have indeed suggested that auxin trans- PIN1 port mediates the lateral distribution of auxin [10,29]. The search for molecular support for this concept brought about the identi®cation of PIN3 [10]. PIN3 is involved in hypocotyl and root tropisms, and is localized in the PIN2 PIN2 lateral endodermis of shoots, where it is perfectly posi- tioned to regulate auxin redistribution in the lateral direction [10]. The demonstration of a defect in lateral auxin redistribution in pin3 mutants, a more direct proof PIN4 of this scenario, may be dif®cult as the rather subtle tropism defects of pin3 suggest that one or more other PIN proteins may functionally replace PIN3.

In roots, unlike in shoots, the locations of stimulus per- PIN3 ception (in the root cap) and growth response (in the elongation zone) are remote from each other [28].Wedo not know exactly where the asymmetry in auxin distribu- tion is established in roots, but experiments using the DR5 reporter suggest that lateral auxin redistribution has already taken place in the root cap [26,29]. From there, auxin is translocated basipetally in an auxin ef¯ux-and in¯ux-dependent manner [3,6,12,28,30]. AUX1 probably facilitates the uptake of auxin into the lateral root cap and Current Opinion in Plant Biology epidermis region, and PIN2 probably mediates its direc- Immunolocalization of the PIN3 protein in the Arabidopsis root apexand tional translocation towards the elongation zone (see probable routes of (white arrows). PIN3 (in green) Figure 3). The next important question is that of how is localized symmetrically in columella cells and apparently mediates auxin transport is activated and regulated by a stimulus lateral auxin distribution to all sides of root cap. After the root is turned such as gravity. Gravity is perceived by the sedimentation by 90 degrees away from vertical (i.e. after a gravistimulus is applied), of starch-containing organelles (i.e. statoliths) in the PIN3 rapidly relocates to the bottom side of columella cells (inset), and thus probably regulates auxin flux to the lower side of root. Auxin is columella root cap and in shoot endodermis [28]. The further transported through lateral root cap and epidermis cells presence of PIN3 in these cells raises the intriguing basipetally by a PIN2-dependent route (polar localization of PIN2 at the possibility that gravity perception and auxin redistribu- upper side of cells is depicted in blue) This basipetal transport also tion are coupled via PIN3 [10]. This scenario has been requires AUX1-dependent auxin influx. AUX1 is present in the same cells as PIN3 and PIN2. Auxin is supplied to the root cap by the tested in gravistimulated Arabidopsis roots. Under normal PIN1- and PIN4-dependent acropetal route (which is depicted in white). conditions, most of the PIN3 protein is located symme- trically at the plasma membranes of columella cells. After gravistimulation, PIN3 changes its position within two minutes and relocates, presumably towards the new bot- Morris and coworkers [31,32] envisioned the rapid intra- tom of the cells [10]. PIN3 is thus ideally placed to cellular turnover of at least part of the auxin ef¯ux mediate an auxin ¯ux towards the lower side of root (see complexes, even before the molecular components of Figure 3, inset). Interestingly, the auxin in¯ux compo- this complex were identi®ed. These ideas have been nent AUX1 also shows strong subcellular dynamics in the corroborated by the demonstration that PIN1 and columella cells [12]. It is tempting to speculate that PIN3 continuously cycle in membrane vesicles along AUX1 mediates an in¯ux of auxin into the columella the between the plasma membrane after gravity stimulation, thereby creating a temporary and the endosome [10,33]. If a decision about the pool of auxin that is needed for asymmetric relocation of targeting of PIN proteins takes place after each inter- auxin in a PIN3-dependent pathway. nalization event, such a recycling mechanism would be far more ¯exible than a mechanism involving a sequence of But what mechanism enables the rapid subcellular relo- degradation!new protein synthesis!new targeting. It cation of PIN3? Elegant physiological experiments by would provide a means of rapid retargeting and would

Current Opinion in Plant Biology 2003, 6:7±12 www.current-opinion.com Auxin transport Friml 11

explain why plants invest so much energy in the contin- changes the choreography of the intracellular movement uous recycling of proteins that are, in principle, only of auxin transport proteins. This enables plants to mod- needed at the plasma membrane. There is only circum- ulate the direction of auxin ¯uxes and thus auxin dis- stantial evidence that the relocation of PIN3 regulates tribution, which in turn triggers the appropriate response. auxin redistribution, which leads to gravitropic bending. Substantial work is needed to outline the details of this This evidence comes mainly from correlation of experi- roughly shaped concept. More general validity should be ments that have shown that functional PIN3 is required demonstrated by extending this model to developmental for proper , and that any disruption of the processes other than root gravitropism. Loose ends, such actin-dependent cycling of PIN3 (for instance by `auxin as the connection between perception and protein reloca- ef¯ux inhibitors', by the vesicle-traf®cking inhibitor bre- tion, as well as the downstream signaling of auxin dis- feldin A or by actin depolymerization) results in gravi- tribution remain to be cleared up. Similarly, the input and tropism defects [10,28,33]. Rigorous testing of the integration of endogenous signals, which certainly mod- hypothesis that the relocation of PIN3 mediates auxin ulate the whole process, are topics for future research. redistribution would require the replacement of PIN3 by Plants are patient and some of them long living, main- a non-relocating but otherwise functional version, and the taining our hope that the beauty of the linden tree at subsequent analysis of auxin redistribution. The presence Buchlov castle will never be diminished in our eyes, even of both statoliths and PIN3 in the shoot endodermis after losing its innocence. suggests that a similar mechanism, involving the reloca- tion of PIN3 and/or other PIN proteins, operates during Acknowledgements shoot tropisms, but this remains to be demonstrated. We are grateful to Eva Benkova and Dolf Weijers for helpful discussions and critical reading of the manuscript. We are deeply indebted to Karen Cornelis Recent studies on two other endodermis proteins, for her insuperable lesson on the use of articles in English. We acknowledge SHOOT GRAVITROPIC2 (SGR2) and SGR4, suggest support by the VolkswagenStiftung. a connection between membrane traf®c, vacuole organi- zation and shoot gravitropism. However, it seems that the References and recommended reading sgr2 and sgr4 mutations interfere with gravity perception Papers of particular interest, published within the annual period of review, have been highlighted as: rather than with auxin redistribution as they are defective in the sedimentation of statoliths and display normal  of special interest  of outstanding interest phototropism [34,35]. 1. Thimann KV: Hormone Action in the Life of the Whole Plant. One important question that remains is that of how the Amherst: The University of Massachusetts Press; 1977. sedimentation of statoliths is connected to PIN3 reloca- 2. Darwin C, Darwin F: Das BewegungsvermoÈ gen der Planze.In Darwin's Gesammelte Werke vol 13. Stuttgart: Schweizer-bart'sche tion. Classical, as well as recent models, suggest that the Verlagsbuchhandlung; 1881. [Title translation: The power of actin cytoskeleton is reorganized due to statolith sedi- movement in plants.] mentation [36,37]. Thus, the actin-dependent intracellu- 3. Friml J, Palme K: Polar auxin transport Ð old questions and new lar traf®c of PIN3 would be redirected along the concepts? Plant Mol Biol 2002, 49:273-284. sedimentation routes, and PIN3 would preferentially 4. Rubery PH, Sheldrake AR: Carrier-mediated auxin transport. accumulate at the cell bottom. It is probable that reality Planta 1974, 118:101-121. is more complex than our simpli®ed, mechanistic idea and 5. Raven JA: Transport of indole acetic acid in plant cells in relation to pH and electrical potential gradients, and its the exact elucidation of this issue remains a challenge for signi®cance for polar IAA transport. New Phytol 1975, future investigations. 74:163-172. 6. Bennett MJ, Marchant A, Green HG, May ST, Ward SP, Millner PA, Conclusions Walker AR, Schulz B, Feldmann KA: Arabidopsis AUX1 gene: a permease-like regulator of root gravitropism. Science 1996, Auxin distribution contributes to the plasticity of plant 273:948-950. development and mediates a wide array of responses by 7. Luschnig C, Gaxiola R, Grisa® P, Fink G: EIR1, a root speci®c which plants adjust their growth to changes in environ- protein involved in auxin transport, is required for gravitropism ment. In an effort to meet all of these demands, auxin in Arabidopsis thaliana. Genes Dev 1998, 12:2175-2187. appropriates the characteristics of a hormone, such as 8. GaÈ lweiler L, Guan C, MuÈ ller A, Wisman E, Mendgen K, Yephremov long-distance effects and distribution through the phloem A, Palme K: Regulation of polar auxin transport by AtPIN1 in Arabidopsis vascular tissue. Science 1998, 282:2226-2230. vasculature, as well as features of a morphogen, having a 9. MuÈ ller A, Guan C, GaÈ lweiler L, TaÈ nzler P, Huijser P, Marchant A, gradient-dependent in¯uence on patterning in the root Pary G, Bennet M, Wisman E, Palme K: AtPIN2 de®nes a locus of meristem. Yet, classifying the effects of auxin has not Arabidopsis for root gravitropism control. EMBO J 1998, signi®cantly advanced our understanding of its molecular 17:6903-6911. mechanism. Now that the intracellular dance of auxin 10. Friml J, Wisniewska J, Benkova E, Mendgen K, Palme K: Lateral  relocation of auxin ef¯ux regulator AtPIN3 mediates tropism in transport components have been correlated to tropisms, a Arabidopsis. Nature 2002, 415:806-809. picture of how the plant shapes itself is beginning to The authors present their characterization of a novel member of the family of auxin ef¯ux regulators, AtPIN3. pin3 mutants are defective in emerge. In the scenario presented in this review, a plant differential growth, including root and hypocotyl tropisms. PIN3 is perceives cues from its surroundings and accordingly positioned at the lateral side of endodermis cells, providing a candidate www.current-opinion.com Current Opinion in Plant Biology 2003, 6:7±12 12 Growth and development

component of the lateral auxin-transport system. PIN3 undergoes rapid 24. Uggla C, Mellerowicz EJ, Sundberg B: Indole-3-acetic acid subcellular movement in vesicles along actin ®laments. After stimulation controls cambial growth in Scots pine by positional signalling. by gravity, PIN3 in columella cells relocates towards the presumable Plant Physiol 1998, 117:113-121. new cell bottom, providing a means for lateral auxin redistribution after gravity stimulation. 25. Casimiro I, Marchant A, Bhalerao RP, Beeckman T, Dhoog S, Swarup R, Graham N, Inze D, Sandberg G, Casero PJ, Bennett M: 11. Friml J, Benkova E, Blilou I, Wisniewska J, Hamann T, Ljung K, Auxin transport promotes Arabidopsis lateral root initiation.  Woody S, Sandberg G, Scheres B, JuÈ rgens G, Palme K: AtPIN4 Plant Cell 2001, 13:843-852. mediates sink driven auxin gradients and patterning in Arabidopsis roots. Cell 2002, 108:661-673. 26. Sabatini S, Beis D, Wolkenfelt H, Murfett J, Guilfoyle T, Malamy J, This work focuses on an analysis of the auxin ef¯ux regulator PIN4. PIN4 Benfey P, Leyser O, Bechtold N, Weisbeek P, Scheres B: An is asymmetrically localized towards the auxin response maximum in auxin-dependent distal organizer of pattern and polarity in the embryonic and seedling root . Atpin4 mutants are defective in Arabidopsis root. Cell 1999, 99:463-472. the establishment and maintenance of an endogenous auxin gradient, 27. Went FW: Re¯ections and speculations. Annu RevPlant Physiol and display patterning defects in both developing and mature roots. The 1974, 25:1-26. authors propose a role for AtPIN4 in generating a sink for auxin below the quiescent center that is essential for auxin distribution and patterning. 28. Chen R, Guan C, Boonsirichai K, Masson PH: Complex physiological and molecular processes underlying root 12. Swarup R, Friml J, Marchant A, Ljung K, Sandberg G, Palme K, gravitropism. Plant Mol Biol 2002, 49:305-317.  Bennett M: Localisation of the auxin permease AUX1 suggests two functionally distinct hormone transport pathways operate 29. Rashotte AM, DeLong A, Muday GK: Genetic and chemical in the Arabidopsis root apex. Genes Dev 2001, 15:2648-2653. reductions in protein phosphatase activity alter auxin This work localizes the putative auxin in¯ux carrier AUX1 in the transport, gravity response, and lateral root growth. Plant Cell Arabidopsis root apex. The asymmetric localization of AUX1 at the 2000, 13:1683-1697. upper side of protophloem cells suggests a connection between AUX1 and the phloem-based IAA transport pathway. Auxin analyses show that 30. Parry G, Delbarre A, Marchant A, Swarup R, Napier R, IAA content in the roots of the aux1 mutant is lower than that in wildtype Perrot-Rechenmann C, Bennett MJ: Novel auxin transport roots, consistent with a phloem-unloading function for AUX1. AUX1 inhibitors phenocopy the auxin in¯ux carrier mutation aux1. localization to columella and lateral root cap tissues rationalizes the Plant J 2001, 25:399-406. agravitropic phenotype of aux1. 31. Morris D, Robinson J: Targeting of auxin carriers to the 13. Noh B, Murphy AS, Spalding EP: Multidrug resistance-like genes plasma membrane: differential effects of brefeldin A on the of Arabidopsis required for auxin transport and auxin-mediated traf®c of auxin uptake and ef¯ux carriers. Planta 1998, development. Plant Cell 2000, 13:2441-2454. 205:606-612. 14. Murphy AS, Hoogner KR, Peer WA, Taiz L: Identi®cation, 32. Robinson JS, Albert AC, Morris DA: Differential effects of puri®cation, and molecular cloning of N-1-naphthylphthalmic  brefeldin A and cycloheximide on the activity of auxin acid-binding plasma membrane-associated aminopeptidases ef¯ux carriers in Cucurbita pepo L. J Plant Physiol 1999, from Arabidopsis. Plant Physiol 2002, 128:935-950. 155:678-684. The authors use inhibition of Golgi-mediated protein traf®c and protein 15. Kendrew J: The Encyclopedia of Molecular Biology. Oxford: synthesis, combined with measurements of the rate of IAA ef¯ux, to Blackwell Science; 1994. demonstrate that an essential protein component of the auxin ef¯ux carrier system is targeted to the plasma membrane. The protein is 16. Ljung K, Bhalerao RP, Sandberg G: Sites and homeostatic targeted through the secretory system and turns over rapidly without a  control of auxin biosynthesis in Arabidopsis during requirement for concurrent protein synthesis. vegetative growth. Plant J 2001, 28:465-474. The distribution and biosynthesis of IAA was investigated during early 33. Geldner N, Friml J, Stierhof YD, JuÈ rgens G, Palme K: Auxin plant development in Arabidopsis. The youngest leaves exhibit the  transport inhibitors block PIN1 cycling and vesicle highest relative capacity for IAA synthesis. However, all other parts of the traf®cking. Nature 2001, 413:425-428. plant, including the cotyledons, expanding leaves and root tissues, are The polar localization of PIN1 in root cells results from its rapid actin- able to synthesize IAA de novo. dependent cycling between the plasma membrane and endosomal compartments. Auxin-transport inhibitors block PIN1 cycling and inhibit 17. Novoplansky A, Cohen D, Sachs T: Ecological implications of the traf®cking of other membrane proteins that are unrelated to auxin correlative inhibition between plant shoots. Physiol Plant 1989, transport. In addition, brefeldin A mimics the physiological effects of 77:136-140. auxin transport inhibitors. These data suggest that PIN1 cycling is 18. Bhalerao RP, Eklof J, Ljung K, Marchant A, Bennett M, Sandberg G: required for auxin transport and that auxin ef¯ux inhibitors act as Shoot-derived auxin is essential for early lateral root inhibitors of vesicle traf®cking. emergence in Arabidopsis seedlings. Plant J 2002, 29:325-332. 34. Kato T, Morita MT, Fukaki H, Yamauchi Y, Uehara M, Niihama M, 19. Baker DA: Long-distance vascular transport of endogenous Tasaka M: SGR2, a phospholipase-like protein, and ZIG/SGR4, hormones in plants and their role in source:sink regulation. a SNARE, are involved in the shoot gravitropism of Arabidopsis. Israel J Plant Sci 2000, 48:199-203. Plant Cell 2002, 14:33-46. 20. Marchant A, Bhalerao R, Casimiro I, Eklof J, Casero PJ, Bennett M, 35. Morita MT, Kato T, Nagafusa K, Saito C, Ueda T, Nakano A, Tasaka  Sandberg G: AUX1 promotes lateral root formation by  M: Involvement of the vacuoles of the endodermis in the early facilitating indole-3-acetic acid distribution between sink and process of shoot gravitropism in Arabidopsis. Plant Cell 2002, source tissues in the Arabidopsis seedling. Plant Cell 2002, 14:47-56. 14:589-597. The endodermis-speci®c expression of SGR2 and ZIGZAG (ZIG) The authors present data that support the contribution of the putative complement the abnormalities in sedimentation and shoot auxin in¯ux carrier AUX1 to loading of IAA from source tissues in leaves gravitropism of the sgr2 and zig mutants, respectively. ZIG encodes into the vascular transport system. The data also support a role for AUX1 AtVTI11, which is a SNARE involved in vesicle transport to the vacuole. in unloading IAA in the primary root apexand lateral root primordia. The SGR2 encodes a phospholipase-like protein and localizes to the authors suppose that auxin supplied from the apical source by this vacuole. These observations indicate that the vacuole participates in transport route mediates the development of lateral roots. the early events of shoot gravitropism. 21. Wolpert L: Principles of Development. Oxford: Elsevier Science; 36. Baluska F, Hasenstein KH: Root cytoskeleton: its role in 1998. perception of and response to gravity. Planta 1997, 203(Suppl):S69-S78. 22. Teleman AA, Strigini M, Cohen SM: Shaping morphogen gradients. Cell 2001, 105:559-562. 37. Yoder TL, Zheng Hq H, Todd P, Staehelin LA: Amyloplast sedimentation dynamics in maize columella cells support a 23. Entchev EV, Gonzalez-Gaitan MA: Morphogen gradient new model for the gravity-sensing apparatus of roots. Plant formation and vesicular traf®cking. Traf®c 2002, 3:98-109. Physiol 2001, 125:1045-1060.

Current Opinion in Plant Biology 2003, 6:7±12 www.current-opinion.com